On the tetragonality of the room-temperature ferroelectric phase of barium titanate, BaTiO₃

The room-temperature phase of the important ferroelectric material barium titanate, BaTiO₃, was re-investigated by single-crystal X-ray diffraction on a sample grown by the top-seeded solution growth method, with the intention of demonstrating once again that the structure has tetragonal symmetry consistent with the space-group assignment P4mm and thus resolving recent controversy in the scientific community and literature [Yoshimura, Kojima, Tokunaga, Tozaki & Koganezawa (2006). Phys. Lett. A, 353, 250–254; Yoshimura, Morioka, Kojima, Tokunaga, Koganezawa & Tozaki (2007). Phys. Lett. A, 367, 394–401]. To this end, the X-ray diffraction pattern of a small (341 µm³) sample of top-seeded solution-grown BaTiO₃ was measured using an Oxford Diffraction Gemini CCD diffractometer employing Mo Kα radiation and an extended 120 mm sample-to-detector distance. More than 10⁴ individual diffraction maxima observed out to a maximum resolution of 0.4 Å were indexed on two tetragonal lattices. These were identical to within the standard deviations on the lattice parameters and were related to each other by a single rotation of 119.7° about the [11] direction of the first tetragonal lattice (the major twin component), although the actual twinning operation that explains the observed diffraction pattern both qualitatively and quantitatively is shown to be conventional 90° twinning by the m[101] operation. Importantly, it is not necessary to invoke either monoclinic symmetry or a coexistence of tetragonal and monoclinic phases to explain the observed diffraction data.